The process involved in the manufacture of plastic articles through injection molding requires filtering the plastic melt prior to injection. The filtration of the hot melted plastic raises a number of problems due to its viscosity and the need for accurate temperature control. The heavy viscous nature of melted plastic requires smooth flow at all times to avoid dead end spots and premature solidification. The filtration process must be substantially continuous and uninterrupted for the same reason; i.e., a static condition for an excessive period of time may cause premature solidification. At the same time, the temperature must be maintained within a fairly narrow range. Excessive temperature will burn the plastic and cause degradation, whereas temperatures which are too low compound the premature solidification problem.
Once the plastic has been raised to the proper temperature, it must be used relatively quickly and not allowed to exist in a stagnant state. However, the requirement for substantially continuous movement operates against the need to periodically clean the filtration apparatus, which eventually becomes clogged from the filtration of contaminants and other particulate matter. It is difficult to accomplish an objective of manufacturing high quality plastic articles when the filtering apparatus necessitates a shutdown of the operation while the filter screen is cleaned or replaced.
The filter purging or cleaning operation raises an additional problem because it invariably involves exposing the plastic flow system to air when the filtration apparatus is disassembled to service or replace the filter screen. Once air is introduced into the system, the flow of melted plastic must be run for a period of time to insure that all air bubbles or pockets have been discharged. This is not only time consuming, but also wasteful of the raw plastic, which is ordinarily discharged and lost until the air bubbles leave the system.
Thus, intermittent operation caused by periodic shutdown of the filtration apparatus for servicing, and the introduction of air into the plastic melt when the filter is serviced, create a difficult problem which with conventional devices is solved by consuming time and wasting raw product.